A crank mechanism is known as a Mechanism that transmits reciprocating motion by converting the reciprocating motion into rotational motion. For example, engines, compressors and the like include a piston provided reciprocatively inside a cylinder, a coupling rod rotatably connected to the piston, and a crankshaft extending in a direction perpendicular to the direction of reciprocating movement of the piston. The other end of the coupling rod is rotatably connected to a crank pin provided eccentrically with respect to the crankshaft. When the piston reciprocates inside the cylinder, the reciprocating motion is converted into rotational motion of the crankshaft by oscillations of the coupling rod and eccentric rotation of the crankshaft.
In the crank mechanism configured as described above, normally the coupling rod is rotatably connected to the piston via a piston pin and, when power is transmitted, is translated while oscillating about the piston pin. Thus, a force in a rotation direction acts on the piston, causing a frictional loss in a wedge effect shape on a cylinder inner surface at two locations, an outer circumferential portion at a top edge and an outer circumferential portion at a bottom edge of the piston. Normally, smooth reciprocating motion of the piston is enabled by reducing the frictional loss by using a lubricant. However, when a large piston is used, oil may run out, which manifests itself as a sticking phenomenon.
To reduce sticking by such a frictional loss, a driving mechanism provided with a cross head between the piston and coupling rod or using a short piston for a small engine has been proposed.
However, while it is possible to increase the degree of sealing of the piston by providing a cross head, a frictional loss changing every 180° is caused by a wedge effect at two locations also in the cross head. Thus, while a reciprocating motion is produced as a motion, a loss is caused by vibration derived from the reciprocating motion.